Key questions in neuroscience are: how are complex neural circuits assembled in young animals and how do they process information in adults? The retina may be the first part of the mammalian brain for which satisfactory answers to these questions will be obtained. The retina is about as complex as any other part of the brain, but it has several features that facilitate analysis: it is accessible, compact, and structurally regular, and we already know a lot about what it does. Visual information is passed from retinal photoreceptors to interneurons to retinal ganglion cells (RGCs) and then on to the rest of the brain. Each of ~25 types of RGC responds to a visual feature –for example motion in a particular direction– based on which of the ~70 interneuronal types synapse on it. To understand how these circuits form, we mark retinal cell types transgenically, map their connections, seek recognition molecules that mediate their arrangement and synaptic connectivity, and use genetic methods to assess the structural and functional consequences of removing or swapping them. We believe that our methods and results will be useful in tackling less accessible parts of the brain such as the cerebral cortex.